Atmospheric and Climate Sciences, 2014, 4, 385-393 Published Online July 2014 in SciRes. http://www.scirp.org/journal/acs http://dx.doi.org/10.4236/acs.2014.43038

How to cite this paper: Iskander, S.M., Rajib, M.A. and Rahman, M.M. (2014) Trending Regional Precipitation Distribution and Intensity: Use of Climatic Indices. Atmospheric and Climate Sciences, 4, 385-393. http://dx.doi.org/10.4236/acs.2014.43038

Trending Regional Precipitation Distribution and Intensity: Use of Climatic Indices Syeed Md Iskander1*, Mohammad Adnan Rajib2, Md Mujibur Rahman3 1Department of Civil and Environmental Engineering, Washington State University, Pullman, WA, USA 2School of Civil Engineering, Purdue University, West Lafayette, IN, USA 3Department of Civil Engineering, Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh Email: *syeedmd.iskander@email.wsu.edu Received 24 May 2014; revised 12 June 2014; accepted 5 July 2014

Copyright © 2014 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

Abstract Trends and variability of annual precipitation total, annual number of rainy days and two climate change related precipitation indices named Simple Daily Intensity Index (SDII) and Precipitation Concentration Index (PCI) have been investigated in this study. The analysis was based on daily and monthly precipitation data of 35 observatory stations all over Bangladesh for the study period of 1971-2010. Mann Kendall test was performed to detect the trend and Sen’s slope method to de-termine the magnitude of change. The results indicate statistically significant (95% confidence level) negative trend in 4 stations and significant positive trend in 2 stations for annual precipita-tion total. Significant positive trend in 9 stations for annual number of rainy days, significant neg-ative trend in 6 stations for SDII and for PCI, and significant negative trend in 6 stations were found all over Bangladesh in this study. The values of PCI indicate strongly irregular precipitation distribution in South Eastern Region (SER) and mostly irregular distribution in other regions. On the other hand values of SDII indicate strong precipitation intensity in SER and mostly moderate intensity in other regions all over the country.

Keywords Bangladesh, Climate Change Indices, Precipitation, PCI, SDII

*Corresponding author.

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1. Introduction Climate variability and its unfriendly concomitants are of growing concern worldwide. The climate of Bangla-desh is undergoing changes and extremes are becoming more unprecedented every year. Floods, cyclones or droughts are often in its colossal mood here, which are aggravated by climate change and its variability being experienced more frequently in Bangladesh than ever before. Change of precipitation trends is in its way be-cause of climate change. According to the intergovernmental panel on climate change (IPCC) report [1], Ban-gladesh will experience 5% to 6% increase of rainfall by 2030. Bangladesh has been termed by IPCC as one of the most vulnerable countries in the world, which is prone to climate change. Global warming induced changes in temperature and rainfall are already evident in many parts of the world, as well as in this country [2]. High intensity of rainfall and uneven temporal and spatial distribution is creating flooding and longer dry spells are evoking droughty conditions [3]. As a whole, these have profound impact on agriculture [4] and food security of the country will be at risk [5].

In the past three decades, higher Precipitation Concentration Index (PCI > 20) has been recorded in semi-arid and tropical humid environments in the cardamom hill slopes, south western ghats, India, which implies very long dry periods for up to 3 - 5 months [6]. During the last 50 years, Simple Daily Intensity Index (SDII) values for the countries of the western Indian Ocean have been showing significant negative trend for most of the ob-servatory stations and positive trend in only a few of the stations [7]. Specifically for Bangladesh, although a number of studies have been carried out on average precipitation and temperature pattern [8]-[11], research on intensity and distribution of rainfall has hardly been done.

2. Data and Methodology Daily and monthly precipitation data of 35 BMD (Bangladesh Meteorological Department) stations (Figure 1) were used for this study. For the convenience of analysis, whole study area was divided into five regions as North Western Region (NWR), North Eastern Region (NER), Central Region (CR), South Western Region (SWR) and South Eastern Region (SER) (Figure 1).

The Simple Daily Intensity Index (SDII) [7] [12] is defined as follows: Let RRwj be the daily precipitation total for the wet days W (RR ≥ 1 mm) in period j. If W is the number of

the wet days within the period j, then: W

wjW 1j

RRSDII

W== ∑ (1)

The Precipitation Concentration Index [6] [13], being proposed as an indicator of rainfall concentration and rainfall erosivity [14], was calculated on an annual scale for each station according to Equation (2):

( )

12 2ii 1

2t

PPCI 100

P== ×∑ (2)

where Pi is the monthly precipitation in month i and Pt is the annual precipitation total. Table 1 lists different value ranges of SDII and PCI with their relative significances.

Mann-Kendall test [15] [16] was used to analyze the trends in annual rainfall total, annual number of rainy days, SDII and PCI; in addition, the Sen’s slope method [17] was applied to determine the magnitude of change. Confidence level of 95% was taken as threshold to classify the significance of positive and negative trends.

3. Results and Discussion SDII and PCI both are extreme precipitation indices which measure intensity and distribution uniformity of rainfall events respectively. SDII depends on annual rainfall amounts and annual number of rainy days. There-fore, trends of both annual rainfall amounts and annual number of rainy days have effects on trends of SDII. Mann-Kendall trend test values for annual rainfall amounts, annual number of rainy days, SDII and PCI are shown in Table 2 and Figure 2. Results show insignificant negative trends in 15 stations, significant negative trends in 4 stations, insignificant positive trends in 14 stations and significant positive trends in 2 stations for annual rainfall amounts. For annual rainy days, the numbers are 11, 0, 15, and 9; for SDII the numbers are 17, 6, 12, and 0 and for PCI the numbers are 15, 6, 14, and 0 respectively.

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(a) (b)

Figure 1. (a) Study area with regions: Bangladesh (b) BMD stations.

Figure 2. Summary of the results obtained for the trends of the four study parameters using Mann-Kendall test.

Table 1. Significance of SDII and PCI values [6] [7] [12] [13].

SDII Value Significance (Intensity) PCI Value Significance

(Temporal Distribution)

To 15 inclusive Low intensity To 10 inclusive Uniform distribution

Over 15 to 20 Moderate intensity Over 10 to 15 Moderate distribution

Over 20 to 25 High intensity Over 15 to 20 Irregular distribution

Over 25 to 30 Strong Intensity Over 20 Strongly irregular distribution

Over 30 Very strong intensity

0

5

10

15

20

25

30

35

Annual Rainfall (mm) Annual Number of Rainy Days

SDII(mm/day) PCI

No.

of S

tatio

ns

Negative but not significant Negative and significant

positive but not significant Positive and significant

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Table 2. Trends of annual rainfall total, annual number of rainy days, SDII and PCI for the study period at different BMD stations.

Region Stations Trends (unit/year)

Annual Rainfall Total (mm)

Annual Number of Rainy Days

SDII (mm/day) PCI

NWR

Bogra −1.9 0.01 −0.026 −0.064*

Dinajpur −3.69 −0.063 −0.033 −0.085*

Ishurdi −9.48 0.07 −0.128* −0.011

Rajshahi −9.50* 0.059 −0.109* −0.033

Rangpur 10.08 0.167 0.038 −0.064*

Saidpur 15.15 1.25 −0.149 −0.134

CR

Dhaka −1.64 −0.286 0.025 0.018

Tangail −1.93 −0.03 −0.054 0.045

Mymensingh 1.32 0.216 −0.055 −0.071*

Faridpur −9 0.093 −0.108 −0.007

Madaripur −12.25* −0.164 −0.062 0.065

Chandpur −24.41* −0.467 −0.098* −0.092

Comilla 4.86 0.458* −0.008 0.035

Feni −3 −0.25 0.034 −0.008

M. Court −1.03 1.019* −0.165* −0.041

NER Srimangal 9.18 0.889* −0.045 −0.081*

Sylhet −3 0.167 −0.072 −0.028

SER

Rangamati 9.28 0.500* −0.007 −0.046

Kutubdia −8.06 0.609* −0.149* −0.073

Cox’s Bazar 8.32 0.304 0.042 −0.059

Teknaf 19.13* 0.2 0.11 −0.033

Chittagong 11.62 −0.008 0.106 −0.033

Ambagan −6.22 −0.938 0.187 0.175

SWR

Hatya 17.66 0.104 0.117 0.045

Chuadanga 8.87 −0.071 0.086 0.034

Jessore 7.5 0.375* 0.016 0.032

Khulna −3.7 0.025 −0.053 −0.071

Mongla −4.74 −0.29 0.077 0.108

Satkhira 4.35 0.075 0.03 −0.071*

Barisal −2.76 0.068 −0.025 0.044 Bhola −14.08* −0.347 −0.066 0.017

Khepupara 14.18* 0.583* −0.018 0.016 Potuakhali 0.56 0.5 −0.066 0.019 Sitakunda −6.93 0.500* −0.17 −0.007

Sandwip 3.21 0.456* −0.131* 0.021

The asterisk (*) denotes statistically significant figures at 95% confidence level.

Intensive rainfall can be uniform if it occurs all over the time period, but if it occurs within a short instance of the time period then its distribution can be called irregular. Accordingly, when both SDII and PCI shows similar trend, rainfall will become uniform and less intensive (decreasing trend) or irregular and intensive (increasing trend). On the contrary, when the two trends are opposite, outcomes may be uniform and intensive or irregular and less intensive. Thus, in NWR and NER rainfall tends to be uniform and less intensive (Table 2). In other re-

S. M. Iskander et al.

389

gions mixed effects are evident. Six cases of SDII trend change related to the trend of annual rainfall amounts and annual number of rainy

days are detected in this study (shown in Table 3). In case-I, trends of SDII increase although trends of both annual rainfall amounts and annual number of rainy days decrease. It may be due to the milder trend declination of annual rainfall amounts compared to the annual number of rainy days. In case-II, increasing trend of annual rainfall amounts is steeper than that of annual number of rainy days. Cases III and VI (Figures 3(a)-(c)) are

(a)

(b)

(c)

(d)

Figure 3. Temporal evolution of (a) Annual rainfall amounts (mm) (b) Annual number of rainy days (c) SDII (d) PCI in Faridpur (one of the BMD stations) for the study period of 1971-2010 along with 5- year moving average (black line) and linear trend (dotted line).

1000130016001900220025002800

1970 1975 1980 1985 1990 1995 2000 2005 2010

Ann

ual R

ainf

all(m

m)

Year

70

85

100

115

130

145

1970 1975 1980 1985 1990 1995 2000 2005 2010

Ann

ual R

ainy

Day

s

Year

10.0

13.0

16.0

19.0

22.0

25.0

1970 1975 1980 1985 1990 1995 2000 2005 2010

SDII

Year

10

15

20

25

30

1970 1975 1980 1985 1990 1995 2000 2005 2010

PCI

Year

S. M. Iskander et al.

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easy to understand as Equation (1) explains. Cases IV and V are the opposite of cases I and II. The values of PCI shows irregular to strongly irregular precipitation distribution in North Western Region

(NWR) and mostly strong irregularity in South Eastern Region (SER); also irregular distribution is evident in Central Region (CR), North Eastern Region (NER) and South Western region (SWR) (Table 4). While the val-ues of SDII shows moderate to high precipitation intensity in NWR, moderate to strong intensity in CR and Table 3. Cases of SDII change (1971-2010).

Cases Trend of SDII (mm/day)

Trend of Annual Rainfall Amounts (mm)

Trend of Annual Number of Rainy Days

I Increase Decrease Decrease II Increase Increase Increase III Increase Increase Decrease IV Decrease Decrease Decrease V Decrease Increase Increase VI Decrease Decrease Increase

Table 4. Average value of SDII and PCI for the study period at different BMD stations and their significance according to values (Table 1).

Region Stations SDII (mm/day) PCI Comments on SDII Value in

Terms of Intensity Comments on PCI Value in Terms

of Temporal Distribution

NWR

Bogra 16.641 18.66 Moderate Irregular Dinajpur 20.475 20.112 High Strongly Irregular Ishurdi 16.277 17.925 Moderate Irregular

Rajshahi 15.386 18.303 Moderate Irregular Rangpur 20.765 19.023 High Irregular Saidpur 22.158 19.394 High Irregular

CR

Dhaka 17.37 16.227 Moderate Irregular Tangail 16.491 16.255 Moderate Irregular

Mymensingh 18.718 19.345 Moderate Irregular Faridpur 16.787 16.858 Moderate Irregular

Madaripur 17.471 16.684 Moderate Irregular Chandpur 18.865 18.694 Moderate Irregular Comilla 18.226 16.954 Moderate Irregular

Feni 24.735 19.63 High Irregular M.court 26.535 19.356 Strong Irregular

NER Srimangal 17.568 17.04 Moderate Irregular

Sylhet 25.374 16.44 Strong Irregular

SER

Rangamati 19.708 17.856 Moderate Irregular Kutubdia 26.443 20.155 Strong Strongly Irregular

Cox's Bazar 28.681 20.331 Strong Strongly Irregular Teknaf 32.459 21.373 Very strong Strongly Irregular

Chittagong 24.502 19.965 High Irregular Ambagan 25.19 19.298 Strong Irregular

SWR

Hatya 25.298 19.368 Strong Irregular Chuadanga 14.81 19.191 Low Irregular

Jessore 15.522 17.71 Moderate Irregular Khulna 16.196 17.913 Moderate Irregular Mongla 16.012 16.787 Moderate Irregular Satkhira 15.428 18.423 Moderate Irregular Barisal 17.548 17.206 Moderate Irregular Bhola 19.454 17.681 Moderate Irregular

Khepupara 22.648 18.247 High Irregular Potuakhali 21.898 19.7 High Irregular Sitakunda 26.863 18.278 Strong Irregular Sandwip 30.909 19.611 Very Strong Irregular

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NER, mostly strong intensity in SER and low to very strong intensity in SWR (Table 4). Spatial distribution of trends for annual rainfall amounts (mm), annual number of rainy days, SDII and PCI

over the study period of 1971-2010 is shown in Figure 4. Annual rainfall amount (mm) decreases over the cen-tral region (maximum significant negative trend of −24.41 mm/year in Chandpur), and increases in south eastern region (maximum significant positive trend of 19.13 mm/year in Teknaf) (Figure 4(a)) and mixed changes pre-vail over other regions. Annual number of rainy days increases over eastern and northern parts (maximum sig-nificant positive trend of 0.889 day/year in Srimangal) and decreases in central part (Figure 4(b)). SDII shows mixed spatial trends and decrease over northern and southern part; also in central part decreasing pattern is pre-valent (Figure 4(c)). Trends of PCI (Figure 4(d), Figure 3(d)) show decrease over northern and south eastern parts and increase over south western region.

Figure 4. Spatial distribution of trends of (a) Annual rainfall amounts (mm) (b) Annual number of rainy days (c) SDII and (d) PCI over the study period of 1971-2010 (+ for positive, − for negative, o for negative and significant, ∆ for positive and significant change).

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As a whole, this paper shows a guideline to apply climate indices in detecting intensity and temporal distribu-tion of precipitation pattern on regional scale. Our analysis yields some grimacing results for South Eastern Re-gion (SER) and a little part of South Western Region (SWR) (Sitakunda, Sandwip, Hatya) since precipitation is of high intensity and strong irregularity there. Such irregular and highly intensive precipitation can have high erosion-potential. This may reduce the elevation of these regions over mean sea level making it prone to being engulfed by the sea as sea water surface is continually rising due to climate change [1].

Due to erosion, silt charges in river water increase. Consequently conveyance capacity of river gets low as silts settle down to the river bed. This can cause longer lasting floods. Moreover, due to sudden highly intensive rainfall, flash floods and water clogging may occur in cities due to the lack of drainage facilities. Bangladesh is a country which largely depends on its agriculture and hydroelectricity. So the findings of this study can also be useful for developing various decision-support tools in different hydrologic, ecological and agricultural applica-tions.

Acknowledgements Contribution of Bangladesh Meteorological Department (BMD) for providing the necessary meteorological data is acknowledged here.

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